U.S. endurance swimmer Diana Nyad has ended her second attempt in 33 years to swim from Havana, Cuba to the Florida Keys.

After nearly 30 hours in the water, the 61-year-old athlete abandoned her effort Tuesday because of asthma, shoulder pain and battering wind and waves. The swim was expected to take 60 hours and cover 166 kilometers. Nyad was swimming without a shark cage.

Instead, she was protected by an electronic field from equipment towed by kayakers to repel the predators. Five other boats, along with a 45-person support team, accompanied her. Nyad swam without a wetsuit or special equipment.

She had called the swim a "symbolic moment" for increasing understanding between Cuba and the United States, which do not have formal diplomatic relations. Nyad said she does not intend to attempt the swim a third time.

While expressing disappointment at the outcome, Nyad said she does not regret trying to complete the journey. She attempted the same feat back in 1978 when she was 28 years old, but ended it because of high winds and rough seas.

Australian swimmer Susan Maroney completed the swim in 1997 with the help of a shark cage. Maroney was 22-years-old at the time.